Segment Routing (SR) is a technology that implements the Source Routing paradigm. A packet header includes a stack of function identifiers, known as segments, which define an ordered list of functions to be applied to the packet. A segment can represent any instruction, topological or service-based. A segment can have a local semantic to an SR node or global within an SR domain. These functions include, but are not limited to, the forwarding behaviors to be applied successively to the packet, notably destination-based unicast forwarding via a sequence of explicitly enumerated nodes (domain-unique node segments) and links (adjacency segments), and the like. SR allows forcing a flow through any topological path and service chain while maintaining per-flow state only at the ingress node to the SR domain. Segment Routing is described, for example, in Internet Engineering Task Force (IETF) Draft draft-ietf-spring-segment-routing-03, dated May 28, 2015, and entitled “Segment Routing Architecture,” the contents of which are incorporated by reference herein. A particular attraction of Segment Routing is that it obviates the need to install and maintain any end-to-end (e2e) path state in the core network. Only the ingress node for a particular flow needs to hold the segment stack which is applied as the header of every packet of that flow, to define its route through the network. This makes Segment Routing particularly suited to control by a Software Defined Networking (SDN) model.
Segment Routing can be directly applied to Multiprotocol Label Switching (MPLS) with no change in the forwarding plane. A segment is encoded as a MPLS label. An ordered list of segments is encoded as a stack of labels. The segment to process is on the top of the stack. Upon completion of a segment, the related label is popped from the stack. Segment Routing can also be applied to the Internet Protocol (IP) v6 architecture, with a new type of routing extension header—for example, the document published July 2015 as draft-previdi-6man-segment-routing-header (available online at tools.ietforg/html/draft-previdi-6man-segment-routing-header-07). A segment is encoded as an IPv6 address. An ordered list of segments is encoded as an ordered list of IPv6 addresses in the routing extension header. The segment to process at any point along the path through the network is indicated by a pointer in the routing extension header. Upon completion of a segment, the pointer is incremented. Segment Routing can also be applied to Ethernet, e.g., IEEE 802.1 and variants thereof. There are various benefits asserted for SR including, for example, scalable end-to-end policy, easy incorporation in IP and Software Defined Networking (SDN) architectures, operational simplicity, a balance between distributed intelligence, centralized optimization and application-based policy creation, and the like.
Segment Routing is currently only defined for unicast. Segment Routing specifications have explicitly excluded support for native multicast forwarding. Multicast forwarding is valuable for supporting many client services, and especially for supporting client Ethernet services overlaid on MPLS transport [e.g., Virtual Private LAN Services (VPLS), Ethernet Virtual Private Network (EVPN)]. Established IP multicast techniques, such as Protocol Independent Multicast, use signaling over a converged unicast topology to construct multicast trees, but these result in poor restoration performance. Also, the signaling model sits uncomfortably with the SDN control paradigm of Segment Routing. Techniques for multicast in emerging technologies such as Shortest Path Bridging are only applicable to Ethernet. Another emerging technology from the IETF, Bit Indexed Explicit Replication (BIER), requires a completely new data path additional to IP, or to MPLS, or to Segment Routing—it requires a greenfield model. Further, it has very visible scaling thresholds.
Thus, there is a need for multicast systems and methods for Segment Routing, which preserves the advantages inherent therein, avoids signaling, provides optimization, and the like.